1. Field of the Invention
The present invention relates to an exposure apparatus, which exposes a pattern on a substrate through a projection optical system and a liquid, and a device fabricating method.
2. Description of Related Art
Semiconductor devices and liquid crystal display devices are fabricated by a so-called photolithography technique, wherein a pattern formed on a mask is transferred onto a photosensitive substrate. An exposure apparatus used by this photolithographic process includes a mask stage that supports the mask, and a substrate stage that supports the substrate, and transfers the pattern of the mask onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. There has been demand in recent years for higher resolution projection optical system in order to handle the much higher levels of integration of device patterns. The shorter the exposure wavelength used and the larger the numerical aperture of the projection optical system, the higher the resolution of the projection optical system. Consequently, the exposure wavelength used in exposure apparatus has shortened year by year, and the numerical aperture of projection optical systems has also increased. Furthermore, the mainstream exposure wavelength currently is 248 nm KrF excimer laser, but an even shorter wavelength 193 nm ArF excimer is also being commercialized. In addition, as well as resolution, the depth of focus (DOF) is also important when performing an exposure. The following equations respectively express the resolution R and the depth of focus δ.R=k1·λ/NA,  (1)δ=±k2·λ/NA2,  (2)
Therein, λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are the process coefficient. Equations (1) and (2) teach that if the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ narrows.
If the depth of focus δ becomes excessively narrow, then it will become difficult to align the surface of the substrate with the image plane of the projection optical system, and there will be a risk of insufficient margin of focus during the exposure operation. Accordingly, a liquid immersion method has been proposed, as disclosed in, for example, PCT International Publication WO99/49504 below, as a method to substantially shorten the exposure wavelength and increase the depth of focus. This liquid immersion method fills a liquid, such as water or an organic solvent, between the lower surface of the projection optical system and the surface of the substrate, thus taking advantage of the fact that the wavelength of the exposure light in a liquid is 1/n that of in air (where n is the refractive index of the liquid, normally about 1.2 to 1.6), thereby improving the resolution as well as increasing the depth of focus by approximately n times. The contents of the abovementioned international publication is hereby incorporated by reference in its entirety to the extent permitted by the national laws and regulations of the designated status (or elected states) designated by the present international patent application.
Incidentally, the abovementioned related art has the problems discussed below.
The exposure apparatus disclosed in the abovementioned PCT International Publication No. WO99/49504 has a constitution wherein the liquid is supplied and recovered so that an immersion area is formed on one part of the substrate; however, if, in a state after the immersion exposure is completed and the liquid in the immersion areas is not sufficiently recovered, the substrate stage, for example, is moved to a load/unload position (substrate exchange position) in order to unload the substrate on the substrate stage and load a new substrate, then there is a possibility that the liquid remaining (adhered) on the front of the projection optical system, or in the liquid supply nozzle, the recovery nozzle, or the like, may drop onto peripheral devices, for example, the guide surface of the stage, the reflecting mirror for the stage interferometer, and the like.
In addition, if liquid remains on the optical element at the front of the projection optical system, then there is a possibility that adhered residue (a so-called water mark) will be left on the optical element at the front of the projection optical system after this remaining liquid has vaporized, and will adversely affect the pattern formed on the substrate when performing the next exposure process. In addition, it is conceivable that immersion areas will form, even when not performing the exposure process, when using a reference plane member, a fiducial mark member, and the like, disposed around the substrate on the substrate stage, and there is a possibility that the liquid in these immersion areas will not be sufficiently recovered and that adhered residue will be left on these members, or that the liquid remaining on these members will scatter.
Furthermore, it is also conceivable that the liquid from the immersion area on the substrate during exposure will scatter and adhere to peripheral apparatuses, members, and the like. If the liquid scattered from the substrate during exposure adheres to, for example, the reflecting mirror for each of the stage interferometers, then there is a risk that it will degrade the accuracy of the stage position measured by the interferometer.